diff --git a/lib/core/Site.js b/lib/core/Site.js
index c6f1060500e3..91b8209b0bb3 100644
--- a/lib/core/Site.js
+++ b/lib/core/Site.js
@@ -56,6 +56,8 @@ class Site extends React.Component {
       htmlElementProps = {};
     }
 
+    const className = this.props.className || '';
+
     return (
       <html {...htmlElementProps}>
         <Head
@@ -66,7 +68,7 @@ class Site extends React.Component {
         />
         <body
           className={classNames({
-            [this.props.className]: true,
+            [className]: className,
             separateOnPageNav: this.props.config.onPageNav == 'separate',
           })}>
           <HeaderNav
